Extrusion blow molding (EBM) is a well known manufacturing process used for producing hollow parts/bottles/containers from a plastic material. In EBM process, the polymer is melted and extruded into a hollow tube (a parison) and subsequently blow molded in containers of various sizes and shapes. The technology of machine and design as well as processing is quite advanced and matured.
The polymers that are currently used in EBM application include polyethylene, polypropylene, SBC, and PETG. For a polymer to be processed in a typical EBM application it must have the requisite melt strength. Furthermore, the polymer must also possess good stretch-ability in order to make a container of any design. For bottles/containers in packaging application, color and clarity (transparency) are also important characteristics.
PET polyester possesses many of the desirable properties that are required for design and development of wide variety of packaging materials which also include bottles and container for food grade and non food grade items. For example, PET polyester is lighter in weight, unbreakable, transparent, recyclable and it also possesses high gas barrier properties. Furthermore, PET has been widely approved worldwide for packaging food grade items by many regulatory agencies. Because of all these reasons, there is a lot of market potential for PET polyester that is suitable for EBM application. In packaging applications colour and clarity (i.e. transparency) are also the most important desired characteristics.
However, unlike the other polymers mentioned herein above, PET polyester as such can not be used for EBM application, on account of its low melt-strength. High melt strength is required to get required molten polymer flow like a hollow tube in the extrusion process i.e. parison & subsequently blow the parison where polymer is still in the molten state. Besides melt strength, a good stretchability is another important property of the polymer for manufacturing a bottle or a container of any design. When it comes to subjecting a PET polyester to extrusion blow molding for preparation of bottles and container, it has a severe shortcoming. Because of its poor melt strength, conventional PET polyester is not suitable for Extrusion Blow Molding. Attempts have been made to overcome this shortcoming by modifying the PET polyester so as to make it suitable for EBM applications. Also colour and clarity of bottles are important required characteristics in packaging application. Following Patents/applications disclose several methods of preparation of polyester resin compositions that claim suitability of PET for EBM application.
U.S. Pat. No. 4,219,527 discloses a process for blow molding a modified polyethylene terephthalate polymer. U.S. Pat. No. 4,219,527 is particularly silent over the use of chain extending agents for modifying the PET polymer. Furthermore, it is also silent as far as clarity, color and acetaldehyde content of the modified PET polymer is concerned.
US Patent Application no 20080093777 discloses extrudable PET blends using chain extenders during extrusion process of EBM which employs a slow crystallizing polyester copolymer. Also, it is not possible to ensure proper uniform melt quality as the residence time in the extruders is very short to ensure uniform mixing without polymer degradations. One would also need blending equipments on every EBM machine.
Another U.S. Pat. No. 5,523,135 is based on mechanical blending of styrenic copolymer and glycidyl esters of unsaturated acids and vinylic co monomers. This is again a complicated blending process which not only seriously affects colour and clarity but also creates non uniform polymer quality and non uniform melt strength. Furthermore, the process as taught in U.S. Pat. No. 5,523,135 needs additional equipments. U.S. Pat. No. 5,523,135 discloses a process for production of PET in which pre-polymerised PET with a high molecular weight is used in a blending or compounding step. The person skilled in the art would not take into account adding a chain extender into a reaction mixture containing phthalic acid and ethylene glycol due to the high concentration of reactive carboxylic acid groups which the person skilled in the art expects to react immediately with the chain extender. Hence, the teaching of U.S. Pat. No. 5,523,135 is restricted to production of PET by using a chain extender in the melt blending or compounding stage, only. There is no indication that reacting the chain extender prior to the polymerization of the PET is a suitable way to produce PET suitable for an EBM process.
U.S. Pat. No. 5,523,382 discloses process for producing polyester adapted to be extrusion blow molded into articles having improved rheological qualities, that employs 1,4-cyclohexanedimethanol for modifying the properties of the polyester.
U.S. Pat. No. 6,984,694 explains the use of chain extenders to improve properties of virgin, recycled and reprocessed condensation polymers. The process mainly describes the use of the chain extender with various polymers by compounding.
The prior art patents disclosed herein above make use of chain extender by compounding the same with the polymers. When it comes to the manufacture of EBM grade polyester, the conventional method of incorporation of the additives through compounding suffers from several shortcomings. For example, incorporation of the chain extender in the polymer by compounding often leads to the development of formation of gel particles in the polymer melt. Furthermore, color and clarity of the polymer gets deteriorated and still furthermore, impact strength and thermal stability of the polymer also gets seriously affected. An additional process step and additional equipment is needed to conduct chain extension via reactive extrusion of the polymer. In many case, a small portion of chain extenders are sufficient to increase the melt strength, however, direct dosing of small quantities of a reactive additive system is very difficult to do in practice and a masterbatch of the chain extender has to be produced prior to be used in reactive extrusion. The current invention overcomes all this additional costs and process steps.
There is therefore, exists a need for a gel-free PET Polyester composition with high melt strength that has high clarity, better color and gloss, improved drawability and requisite impact strength. It is also desirable at the same time that such co-polyester should also be recyclable, and can be directly used by existing EBM machines without need of any additional process such as blending/compounding.